2 INTRODUCTION TO ZOOLOGY. 



who have gone before us, and take advantage of the materials 

 which their industry has collected. 



The first and most obvious thing to be done is, to fix upon 

 some good distinguishing marks by which the principal groups 

 of animals may be separated from each other. This would, 

 at first sight, appear an easy matter. Thus, birds might be 

 distinguished by the power of flight, and fishes by that of 

 living and swimming in the water. But a little attention 

 would show, that such characteristics would, in both cases, 

 lead to erroneous results. The Bat flies in the air, yet it 

 brings forth its young alive and suckles them as the domestic 

 cat would do. The Whale lives in the sea; but, while in the 

 fish the heart has only two compartments, the blood is cold, 

 and respiration is effected by gills, the Whale has a heart fur- 

 nished, like that of the Ox, with four compartments, the blood 

 is warm, and breathing is carried on by Jungs. The fish 

 deposits its spawn, and the young, when liberated from the 

 eggs, provide for themselves according to their several instincts. 

 The young of the Whale, on the contrary, are brought forth 

 alive, are objects of maternal solicitude,, and are suckled with 

 affectionate assiduity. The Bat, though flying in the air, is 

 not therefore a bird ; the Whale, though swimming in the sea, 

 is not therefore a fish. They both belong to the same division 

 as our large domestic quadrupeds, which, from the circum- 

 stance of their suckling their young, are grouped together by 

 the expressive term " Mammalia." 



It is obvious, therefore, that structure must form the basis 

 of classification. And in the present state of our knowledge, 

 it is no less obvious that arrangements, based on the structure 

 of one particular organ, or one series of organs, to the exclusion 

 of others, would be incomplete, and would lead to error. All 

 organs must be considered, and internal as well as external 

 structure must be examined, before any true systematic 

 arrangement can be attained; and this will be complete, 

 exactly in proportion to the extent and the accuracy of our 

 knowledge. The great object is, to arrange animals in such 

 a way as to exhibit their true affinities to each other, and to 

 embody, with regard to each group, the most comprehensive 

 truths regarding them which the conjoined labours of eminent 

 men have as yet elicited. 



Lamarck, a distinguished French naturalist, proposed ar- 

 ranging all animals according to the presence or absence of a 



